Consolidated Statement of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)
Equity, beginning balance at Dec. 31, 2020	$ 1,959.8	$ 941.1	$ 962.1	$ 56.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	141.0	0.0	141.0	0.0
Other comprehensive income (loss), net of tax expense	(42.3)	0.0	0.0	(42.3)
Common stock transactions:
Common shares purchased and retired	(5.4)	(5.4)	0.0	0.0
Common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	0.4	0.4	0.0	0.0
Stock-based compensation expense	7.5	7.5	0.0	0.0
Common cash dividend declared	(76.2)	0.0	(76.2)	0.0
Equity, ending balance at Sep. 30, 2021	1,984.8	943.6	1,026.9	14.3
Equity, beginning balance at Jun. 30, 2021	1,970.9	941.6	1,005.2	24.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47.1	0.0	47.1	0.0
Other comprehensive income (loss), net of tax expense	(9.8)	0.0	0.0	(9.8)
Common stock transactions:
Common shares purchased and retired	0.0	0.0	0.0	0.0
Common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Stock-based compensation expense	2.0	2.0	0.0	0.0
Common cash dividend declared	(25.4)	0.0	(25.4)	0.0
Equity, ending balance at Sep. 30, 2021	1,984.8	943.6	1,026.9	14.3
Equity, beginning balance at Dec. 31, 2021	1,986.6	945.0	1,052.6	(11.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	116.4	0.0	116.4	0.0
Other comprehensive income (loss), net of tax expense	(496.7)	0.0	0.0	(496.7)
Common stock transactions:
Common shares purchased and retired	(198.9)	(198.9)	0.0	0.0
Common shares issued	1,722.5	1,722.5	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	0.1	0.1	0.0	0.0
Stock-based compensation expense	8.7	8.7	0.0	0.0
Common cash dividend declared	(133.2)	0.0	(133.2)	0.0
Equity, ending balance at Sep. 30, 2022	3,005.5	2,477.4	1,035.8	(507.7)
Equity, beginning balance at Jun. 30, 2022	3,271.9	2,607.9	993.8	(329.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	85.7	0.0	85.7	0.0
Other comprehensive income (loss), net of tax expense	(177.9)	0.0	0.0	(177.9)
Common stock transactions:
Common shares purchased and retired	(133.0)	(133.0)	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	0.0	0.0
Stock-based compensation expense	2.5	2.5	0.0	0.0
Common cash dividend declared	(43.7)	0.0	(43.7)	0.0
Equity, ending balance at Sep. 30, 2022	$ 3,005.5	$ 2,477.4	$ 1,035.8	$ (507.7)